This application claims the priority of German Patent Document 199 24 909.1 filed May 31, 1999 and PCT/DE00 01388 filed May 3, 2000, the disclosures of which a e expressly incorporated by reference herein.
The invention relates to a stressed-skin component made of metal with integrated stiffening profiles, in particular to a skinplate with stringers for aircraft
Usually, aircraft fuselages are made from sheets of skinplate of aluminium alloys, riveted together. When designing aircraft fuselages, specified strength requirements have to be complied with. Apart from static strength and fatigue, these specifications also include compliance with prescribed crack advance behaviour and retained strength behavior. The design of aircraft fuselages reflects compliance with the above-mentioned requirement. For this reason, the design method of skinplates with riveted-on stringers has been applied to stressed-skin components of the exterior skin. This design method above all complies with the strict requirements for rigidity of the fuselage.
In the case of crack damage of the skinplate, with an assumed crack propagation in circumferential direction of the aircraft fuselage, the crack increases under operational loads, thus encountering a stringer. However, if this stringer is connected to the skinplate by riveting or bonding, the crack can usually propagate underneath the stringer without damaging the stringer in the process. Accordingly, there is relatively little impairment of the retained strength of the entire structure, and no failure of the construction is to be feared.
In order to save production costs and operating costs, endeavours have been made to replace riveting of the stringers by weld connections (DE 196 39 667 C1). With this method it must be taken into account that a crack occurring transversely to a welded-on stringer can propagate equally in the skinplate and in the stringer, consequently severely cracking or even through-cracking the stringer. For this reason, the crack advance behaviour and retained strength behaviour of a stressed-skin component damaged in this way, are inferior to those of a riveted stressed-skin component. In addition, when welding the usual stringers by means of laser beam welding techniques used for this, time and again cracks occur in the weld seams; such cracks result from impediments to shrinking during cooling off of the weld seam. The use of weld filler metal to improve the solidification condition, can only partly compensate for this disadvantage
It is the object of the invention to create a cost-effective stressed-skin component made of metal with integrated stiffening profiles, which skin component apart from the required rigidity also provides adequate crack advance behaviour and retained strength behaviour.
According to the invention, this object is met by a stressed-skin component made of metal, comprising a skinplate and stiffening profiles, characterized in that the stiffening profiles in a base area comprise a thicker region of a profile base thickness; in that towards the connection juncture the thicker region has a reduced connection thickness; and in that the ratio of profile base thickness to connection thickness is at least two. Developments of the invention are described herein and in the claims.
Advantageously, the invention applies a geometry-related local reduction in mechanical tension loads at the front of a crack penetrating a stiffening profile of the stressed-skin component. The invention is also based on the recognition that in order to achieve the required rigidity behaviour, the forces which have to be absorbed by the stringer rivets in the case of riveted-on stringers, are relatively small. Often a single row of rivets is adequate for stringer attachment. Therefore, in the case of welded-on stiffening profiles, the depth of the weld seam can be reduced. In this way, shrinkage is improved and a reduction in hot crack formation is achieved.
Reduced depth of the weld seem provides an additional advantage in that increased welding speeds can be achieved, resulting in a reduction in thermal distortion of the stressed-skin components, caused by the weld temperature.
Below, embodiments of the invention are described in more detail by means of the drawing.